In general, natural rubber is frequently used in many fields including industrial products such as tires, rubber belts, rubber rolls, bladders and fenders and sporting goods such as tennis balls, basketballs, soccer balls and volleyballs. In a tire, it is used as a material for various components constituting rubber tires, such as treads, side walls, ply coating rubbers and bead fillers.
In recent years, it has become apparent that protein contained in natural rubber is a cause to bring about an allergic symptom, and a lot of deprotenizing techniques have been proposed.
Natural rubber is more excellent in mechanical characteristics than synthetic rubber and has low so-called tan δ (dynamic loss factor). Accordingly, natural rubber is excellent in a low hysteresis loss property, but it is desired to be improved in a gripping property in a wet state. Further, crude natural rubber (rubber used as a raw material) is inferior in processability and a productivity in the production. This is because protein and phospholipid are bonded to a molecular end of natural rubber, so that proteins themselves and phospholipids themselves are further bonded and associated to form a higher order branched structure. In order to solve such problem, a deprotenizing technique in which a total nitrogen content of natural rubber is reduced to 0.1 mass % or less has been proposed (refer to Japanese Patent Application Laid-Open No. 329838/1994). In conventional deprotenizing treatments, natural rubber latex is repeatedly washed with surfactants, and washing thereof is accompanied with centrifugal process. Further, it is carried out to add a proteolytic enzyme to natural rubber latex to decompose protein and then repeatedly wash it with a surfactant, and protease originating in a bacteria is preferably used as the proteolytic enzyme (refer to Japanese Patent Application Laid-Open No. 56902/1994 and Japanese Patent Application Laid-Open No. 56906/1994).
For example, anionic surfactants and/or nonionic surfactants can be used as a surfactant for deprotenizing treatment. The anionic surfactants include, for example, surfactants of a carboxylic acid base, a sulfonic acid base, a sulfate base and a phosphate base.
On the other hand, it is possible to reduce a branch degree of a higher-order branched structure to improve a processability of natural rubber. However, if a deprotenizing technique is used to completely remove protein, a large part of non-rubber components including protein is removed as well, and the satisfactory processability-improving effect can not be obtained. This reduces an aging resistance of a natural rubber. Further, complete deprotenization reduces entanglement of the molecules themselves originating in a polypeptide bond and the apparent molecular weight to a large extent, which results in reducing the tensile force and the abrasion resistance.